1. Field of the Invention
This invention relates generally to the field of analog integrated circuit design and, more particularly, to the design of oscillators stabilized for temperature and power supply variations.
2. Description of the Related Art
Oscillators play a prominent role in the functionality of a large portion of today's analog and digital systems. Typically, oscillators, also referred to as astable multivibrators, are electronic circuits that convert energy from direct-current sources into periodically varying electrical signals, or voltages. In other words, an oscillator typically operates by utilizing the electrical behavior of its circuit elements to convert a steady state input signal into a periodic, time variant output signal. In some implementations the signal produced by an oscillator may be sinusoidal in appearance, such as a sine wave, in other implementations it may appear as a square wave, triangular wave, or a variety of other repeatable signals. Many of today's integrated circuits that require oscillators, such as timer circuits and Phase-Locked Loops (PLLs), need to include the oscillators on-chip in order to meet cost and area requirements. The behavior of such on-chip oscillators is typically directly or indirectly affected by the technology used to fabricate the integrated circuit. For example, many widely used fabrication processes today are based on complementary metal-oxide-semiconductor (CMOS) technology, where each specific qualified CMOS process varies slightly from another. In addition, the accuracy of oscillators may also be affected by variations in temperature as well as variations in the power supply voltage(s) powering the oscillator.
One common type of oscillator is the relaxation oscillator. Typically a relaxation oscillator achieves its oscillating output by charging a capacitor to some event or switching threshold. The event discharges the capacitor, and its recharge time determines the repetition time of the events or switching. Similarly, an oscillating output could also be achieved by discharging instead of charging the capacitor to reach the event or switching threshold. Typically the capacitor is charged through a resistor, where the values of the resistor and the capacitor, referred to as the RC time constant, determine the rate, or frequency, of the oscillation. For example, decreasing the value of the resistor may increase the oscillation frequency, and increasing the value of the resistor may decrease the oscillation frequency. Typical relaxation oscillators whose frequency is determined by an RC time constant may be prone to temperature and voltage supply variations, since the resistor(s) and capacitor(s) (corresponding to the RC time constant) are temperature dependent, as well as supply dependent with the amplitude of the signals typically affecting the oscillator frequency.
Another widely used type of oscillator is the ring oscillator. A ring oscillator is typically a device comprising an odd number of logic gates with an inverting output, whose output oscillates between two voltage levels, representing logic levels of “1” and “0”. The gates are usually inverters coupled in a chain, with the output of the last inverter in the chain coupled back to the input of the first inverter. Because a single inverter provides an inverted version of its input, the output of a last inverter in a chain composed of an odd number of inverters will be the logically inverted version of the first input. This final output will be asserted a finite amount of time after the first input has been asserted, and providing the inverted (final) output as the feedback to the input will therefore lead to oscillation. A ring oscillator typically only requires a power supply to operate, and once a certain threshold voltage has been reached, oscillations may begin automatically. To increase the frequency of a given oscillator, the applied voltage may be increased, thereby increasing both the frequency of the oscillation and the consumed power, which is generally dissipated as heat. However, any dissipated heat may limit the speed of a given oscillator. Similar to relaxation oscillators, ring oscillators are also prone to temperature and power supply voltage variations, which may affect the accuracy of the oscillator's output frequency.
While various solutions have been implemented to guard oscillators against variations in temperature and supply voltage levels, for example including providing temperature compensation for the logic used in a ring oscillator, supply voltage variations still make it difficult to avoid variations in the oscillator output frequency. Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.